Progress We first introduce the dielectric function of UTMFs to understand their optical properties. The dielectric function of the UTMFs can be described by the Drude model; however, the intrinsic plasmon frequency and inelastic scattering rate need to be corrected.
Conclusions and Prospects The key point in preparing continuous UTMFs is to suppress the island growth of the metal films on the substrate in order to reduce the percolation threshold. Current developments have achieved 1.84 nm thick continuous ultrathin gold films. Further efforts are required to improve the scalability of large-scale UTMFs. Moreover, the relationship between the processing and the corresponding properties is a fundamental scientific issue that requires further investigation.
It would be interesting to prepare nanotailored UTMFs for further study of nano-optical properties in the infrared region. Moreover, the ultrathin nature of UTMFs makes it possible to integrate with nanophotonic waveguides for chip-scale applications of integrated nonlinear optics. As an emerging quasi-two-dimensional material, it has a broad scope in terms of preparation, properties, and applications. With continuous advances in the preparation process, thinner, flatter, and larger-scale UTMFs are expected to be prepared in the future. The unique properties of ultrathin metal films and their applications are also worthy of investigation.
When the thickness of the metallic film is reduced to the nanoscale level, ultrathin metal films (UTMFs) exhibit properties that are remarkably different from those of bulk metals, such as stronger surface plasmons and nonlinear optical responses. However, research progress in this field is slow because of the difficulties in preparing continuous UTMFs owing to the dominant island growth mode of metal thin films at the early stage of growth. With the methods proposed in recent years, such as the seed layer method, organic modification method, co-deposition method, and cryogenic deposition method, researchers have prepared continuous metal films with thicknesses of a few nanometers and found that they have many novel optical properties, such as broadband absorption in the infrared, electrically tuned surface plasmon, and strong nonlinear optical response.
the seed layer method, organic modification method, co-deposition method, and cryogenic deposition method. The seed layer method involves pre-depositing a layer of material (usually using transition metals such as titanium and chromium) with a thickness of 1-2 nm on the substrate as an adhesion layer, which can improve the wettability of the metal on the substrate. The organic modification method usually modifies a layer of silane with specific groups on the substrate, thus increasing the adhesion between the metal and substrate and inhibiting island growth. The co-deposition method involves the preparation of metal films by depositing small amounts of metals such as Al, Ni, Cu, Ti, and Cr on the substrate in a doping-like manner with the target metal at the same time. In contrast, the cryogenic deposition method uses a low-temperature substrate to limit the migration of metal atoms, enabling the preparation of ultrathin metal films without seed layers.
1) The resonant wavelength of the surface plasmon red-shifts as metal films become thinner. Moreover, the surface plasmon frequency of ultrathin metal films can also be electrically tuned, as its body charge density is significantly lower than that of its thick metal film counterpart. 2) The real and imaginary parts of the refractive index of the UTMFs are approximately equal in the mid- and far-IR regions. When its impedance is 188, the impedance matching condition is satisfied, resulting in wavelength-independent broadband absorption (50%). 3) The confining effect in UTMFs quantizes the free electrons of the metal into multiple subbands. Further, the dipole transit moment between the subbands is the individual electron charge multiplied by several nanometers, which is much larger than that of traditional nonlinear crystals. 4) UTMFs have excellent electrical conductivity and mechanical flexibility as well as high transmittance in the visible band, making them suitable for replacing ITO as a flexible transparent conductive material.
